Difference and similarity of dielectric relaxation processes among polyols

Abstract

Complex permittivity measurements were performed on sorbitol, xylitol, and sorbitol-xylitol mixture in the supercooled liquid state in an extremely wide frequency range from 10 microHz to 500 MHz at temperatures near and above the glass transition temperature. We determined detailed behavior of the relaxation parameters such as relaxation frequency and broadening against temperature not only for the alpha process but also for the beta process above the glass transition temperature, to the best of our knowledge, for the first time. Since supercooled liquids are in the quasi-equilibrium state, the behavior of all the relaxation parameters for the beta process can be compared among the polyols as well as those for the alpha process. The relaxation frequencies of the alpha processes follow the Vogel-Fulcher-Tammann manner and the loci in the Arrhenius diagram are different corresponding to the difference of the glass transition temperatures. On the other hand, the relaxation frequencies of the beta processes, which are often called as the Johari-Goldstein processes, follow the Arrhenius-type temperature dependence. The relaxation parameters for the beta process are quite similar among the polyols at temperatures below the alphabeta merging temperature, T(M). However, they show anomalous behavior near T(M), which depends on the molecular size of materials. These results suggest that the origin of the beta process is essentially the same among the polyols.

Systematic study of the glass transition in polyhydric alcohols

“…ρ and RI stand for density and refractive index, respectively. and sorbitol [9]. All the loci for the trihydric alcohols are close to each other, and their curvatures are smaller than those of sugar alcohols.…”

“…3, the peak frequencies of α relaxation of the trihydric alcohols are plotted against reciprocal temperature, together with the previously reported data for threitol, xylitol, TABLE I. Fitting parameters for the VFT equation and several characteristic quantities for trihydric alcohols and sugar alcohols [9]. β KWW (T g ) is KWW exponent at T g .…”

“…The trihydric alcohols with N C = 3 through 7 are examined here [12]. Their properties are also compared with the previously reported results for sugar alcohols, DL-threitol, xylitol, and D-sorbitol [9]. The values of N C and N OH of these materials are listed in Table I.…”

“…1). It has been reported that sugar alcohols exhibit systematic increases in fragility and T g with increasing chain length [7][8][9].…”

“…The temperature dependence of the relaxation time in the supercooled state is often characterized by fragility [3] and T g , and the link between these quantities and chemical structure has been discussed for a long time [4][5][6][7][8][9]. In polymeric materials, Kunal et al have found the qualitative relation between fragility and T g and stiffness of chain: Fragility is dominated by relative flexibility of the side group compared to flexibility of the backbone, whereas T g depends primarily on the backbone flexibility and the side group bulkiness [5].…”